Team claims midrange wireless energy transfer

Portland, Ore. -- The hurdles to wireless power transfer through space have been perceived to be so great that the last serious work on the topic, reported in the 1920s, was inspired by Nikola Tesla's seminal demonstrations circa 1890. But now an MIT physicist claims the obstacles to wireless power transfer are surmountable, at least for distances under 12 feet.

Marin Soljacic described his technology, which he believes could wirelessly recharge almost any battery, in San Francisco last week at the American Institute of Physics' Industrial Physics Forum.

"Tesla and his contemporaries tried very hard to come up with wireless energy transport over long distances," said Soljacic, an assistant professor at the Massachusetts Institute of Technology whose work on the topic netted him the Young Scholar award at last year's Amazing Light: Visions for Discovery symposium. "We have not solved Tesla's energy transfer problem over long distances, but we do think that our technique could potentially be useful for medium-range wireless energy transfer."

Earlier work on wireless power transfer revealed a catch-22. Omnidirectional energy beacons are uneconomical since they waste too much power dispersing energy in all directions. On the other hand, directional energy beams, such as high-energy lasers, need an unobstructed line of sight and could "cook" any objects that get in the way.

The solution, according to Soljacic, is "nonradiative resonant energy transfer," which he says can harness omnidirectional energy beacons without wasting energy, without requiring a clear line of sight and without damaging obstacles in the process. Power from such energy beacons would pass harmlessly through everything but their intended targets, by virtue of resonant power antennas that would be tuned to the power beacon's frequency in a lock-and-key approach.

"We are proposing the use of evanescent tails of oscillatory electromagnetic energy resonan- ces to transfer power wirelessly and nonradiatively over distances of a few meters," said Soljacic, who worked on the design with professor John Joannopoulos, the Francis Wright Davis professor of physics at MIT, and electrical engineer Aristeidis Karalis, a doctoral candidate in MIT's department of electrical engineering and computer science.

The technique is similar to the mechanism that enables a cell phone's resonant antenna to receive only intended conversations, but the antenna's capacitance is large enough to handle the efficient transfer of energy within a range of about 12 feet. Nonradiative fields ensure that little energy is radiated (and lost) into free space. The physics principle at work "is that two same-frequency resonant objects will tend to couple with each other strongly, while only interacting weakly with other, off-resonant environmental objects," said Soljacic.

After Soljacic won the $10,000 Young Scholar award for his proposal, he and his collaborators refined the concept using MIT's electromagnetic-field simulator, which calculates the exact solutions to Maxwell's three-dimensional vector equations, including losses and dispersion. Thus far, simulations of the nonradiative energy transfer technique include one using dielectric disks (of titania, barium tetratitanate or lithium tantalite) and one based on capacitively loaded conducting-wire loops.

"Dielectric disks might work better for nano- and microscale applications, where energy is transferred over very short distances, while conductive loops will probably be better for distances of a few meters," said Soljacic.

Many small devices, such as electric toothbrushes, use magnetic induction for wireless recharging. Induction works by running an electrical current through a coil, with a like-sized coil in the device set at very close range. The magnetic field emitted by the driver coil induces a current in the device's coil to recharge the device. But induction requires that the device to be recharged be set as close by as a metallic connector.

In contrast, Soljacic's technique sets up an evanescent electromagnetic field that causes the receiver's tuned antenna to lock on and slowly begin resonating at the same frequency. Unlike an induction charger, which immediately begins transferring energy at its peak rate, a resonant charger must slowly ramp up the oscillation in the receiver antenna until the peak rate of energy transfer is reached.

The dielectric-disk resonating antenna design, which depends on a purely electrical field, enables energy transfers that could be crafted to be very, very small--even submicron. Thus far, the most suitable design for room-distance applications uses magnetic fields and capacitively loaded conductive loops that appear to be very similar to those used as resonant antennas in cell phones, but with their capacitance increased so that they can transfer power at lower frequencies.

Always onSoljacic believes his technique could be used to keep cell phones, laptops and the like continuously charged. On the factory floor, nonradiative energy transfer might deliver constant power to mobile robots so they would not have to be taken offline for recharging. One could even envision power-beaming devices that could supply wireless power to electric trolleys and buses from an overhead transmission "pipe." On a smaller scale, Soljacic predicts the concept might be used to transfer information in optical interconnects.

Funding for the work was provided by the National Science Foundation and MIT's Materials Research Science and Engineering Center. The team now plans to build prototypes refined for specific applications.